Saturday, September 26, 2009

Mystery Photo for a Slow Saturday

Today's mystery photo is stunning to me, primarily because I didn't think of taking it myself. It shows the death of not one, not two, but three volcanoes.

Oops.

We teachers aren't supposed to fall into the trap of anthropomorphising geologic and other scientific phenomena (i.e. atoms aren't "trying" to reach a lowest energy state in order to be "happy", and volcanoes don't "die"). Let me rephrase the statement:

This photo shows three different volcanic edifices which reached the terminal stage of their development in three different ways.

So, the question of the day: How did these three volcanoes come to be extinct (is that term appropriate? It seems so life-related)? Some context: the picture (by Susan Hayes) was taken during our recent exploration of the southern Cascades in Oregon and northern California.

9 comments:

We're looking at the east side of Llao rock in Crater Lake National Park. Mt. Mazama "died" in a catastrophic, caldera-forming eruption... though I don't think it would be wise to assume it's truly extinct. The two looming over the rim are Diamond Peak (closer) and, I think, Mt Thielsen (farther). I'm pretty sure both of these quit erupting pre-Pleistocene, and have been gouged out by glaciers to the pointy, extinct Cascade volcano form rather than the conical, recently active Cascade volcano form. I'm not sure what the rules are regarding interweb cheating, so I can't say more without checking.

Theilsen is sometime referred to as "the lightning rod of the Cascades;" it's summit has been polished with fulgaritic glass from frequent strikes.

Hmmm. I'm counting just two edifices, though I suppose you might view the nearer one as having multiple stages of growth.

The sharp peak on the horizon is Mount Thielsen, the eroded remnants of a shield volcano located about 13 miles north of Crater Lake. The abrupt cliff rising from Crater Lake is the caldera wall of Mount Mazama. Llao Rock makes up the prominent peak at the left of this view, and was one of the penultimate lava flows in Mount Mazama's construction, filling a glacial valley on the north flank of the mountain.

How did these volcanoes become extinct? Well, simply by exhausting their supply of magma. Extinct is probably the correct term for Thielsen, though dormant might be a better term for Mazama. In any case, the Range they are part of is still very much "alive", though activity in this sector has not been abundant in recent human history. I suspect that anyone around long enough to view the full life-cycle of a volcano would hesitate to declare Mazama dead.

Gee whiz, you didn't waste any time did you? Thanks for the quick responses! I was playing fast and loose with the term "extinct", as Ron and Lockwood point out.

Crater Lake, in the foreground, is a caldera that collapsed in a colossal eruption 7,700 years ago, but eruptions continued as recently as 4,500 years ago (Wizard Island is one of the more recent ones). It would be folly to consider it truly extinct. The original volcano, named (by us) Mt. Mazama was an 11,000-12,000 foot high composite cone.

Mt. Theilsen, the sharp peak in the background, is as noted,a volcano that became extinct, and has been extensively eroded by glacial activity and mass wasting.

The third volcano in the picture is Llao Rock. It forms the solid cliff on the left side of the photo. It is a lava dome, or plug dome, of dacite lava that erupted on the flank of Mt. Mazama about 200 years before the collapse of the caldera. It indicated the evolving of the magma chamber towards a more silicic, explosive phase. The plug filled the crater or valley and spilled over to form the thinner flow on the top of the cliff extending to the center of the photo. It was preceded by a pumice/ash eruption which left a thin white layer under the dacite, visible in the cliff. Since plug domes tend to be one-time affairs most of the time, one could argue that it was extinct as soon as the eruption ended, but there was still a lot of magma below, as the Mazama eruption proved. The dome was sliced in half when Mazama collapsed into the caldera.

Thanks for the rule clarification, and the photo is gorgeous. I meant to ask from where it was taken. At first I was thinking from on the lake itself, then I realized we're looking down onto the lake. South rim with a good telephoto lens? The north rim is so sharp and clear!

Another bit of trivia: on an extraordinarily clear winter day in the early 80's, I could see Mt Theilsen from Marys Peak, 120 miles away. Turning to the north, I could see Mt. Rainier, 180 miles away; all in all, I was looking at about 250 miles of the Cascade crest. Stunning!

Is that the Wineglass of the Wineglass Tuff in the foreground - just to the right of Llao Rock and under the light-yellowish-colored tuff? I once had an exceptional tour of Crater Lake given by Charlie Bacon to a very small group of people.

Silver Fox, that must have been a greet trip with Dr. Bacon! I suspect the only possible Wineglass Tuff would be the light colored material on the very rim. I think most of the thickest tuff deposits were on the east and south rims.

I remember that there is a place on the north to east rim (inside) where a wineglass shape can be seen - it might be more toward the east, but that shape near Llao Rock reminded me of it. Will have to check the next time I'm there. Wish I still had my notes!

About Me

I am a teacher of geology at Modesto Junior College and former president of the National Association of Geoscience Teachers, Far Western Section. I have led field trips all over the western United States, and a few excursions overseas, but my homebase is the Sierra Nevada, the Great Valley, and the Coast Ranges of California.